This invention relates to high speed, sheet-fed printing presses, and more particularly, to a new and improved transfer or delivery cylinder and method of mounting the cylinder in such presses to reduce marking and marring of the freshly printed surface.
In high speed sheet-fed rotary printing presses, freshly printed sheets must be transferred from the impression cylinder to additional processing stations within the press, typically to further impression cylinders for additional printing, or to the delivery station of the press. During such transfer, the wet ink side of the printed sheet must be supported in such a manner that the freshly printed sheet is not marked or marred or the wet ink smeared. One system for insuring that the freshly printed sheet is not marked or marred during transfer is the transfer or delivery cylinder and system marketed by Printing Research, Inc. of Dallas, Texas under the United States registered trademark, "Super Blue". That system, which is made and sold under license, is made in accordance with and operates as described in U. S. Pat. No. 4,402,267, issued Sept. 6, 1983 to Howard W. DeMoore, and comprises, broadly, a right circular transfer or delivery cylinder or wheel mounted on a central drive shaft for rotation thereby, and which is provided with a PTFE (Teflon) coated outer cylindrical sheet support surface over which is loosely mounted a fabric cover referred to in the trade as a "net". It is believed that the loosely mounted net attaches and clings to the printed sheet as the sheet is supported by the cylinder whereby any relative motion between the sheet and cylinder takes place between the surface of the net and the PTFE coated surface of the cylinder so that marking and marring of the printed sheet does not occur.
Typically, high speed sheet fed presses are used to print a wide variety of sheet thickness, generally from very thin "onion skin" type paper having minimum thickness on the order of 0.002 inches (0.05mm) to relatively thick cardboard materials having maximum thickness on the order of 0.034 inches (0.86mm). In order to accommodate the wide variety of sheet thickness, the clearance between the nip of the impression cylinder and the transfer or delivery cylinder must be properly controlled to ensure that each thickness sheet will smoothly transfer from the impression cylinder to the transfer or delivery cylinder without marking or marring.
While the "Super Blue" system marketed by Printing Research, Inc., has met with substantial commercial success, it has been found that some sheet marking or marring of the wet inked surface may occur on the leading or gripper end portion of the printed sheet during initial transfer of the sheet from the impression cylinder. This is believed to be caused by the relatively sharp bend which the sheet must undergo as the sheet grippers, which are associated with the transfer or delivery cylinder, grip the sheet leaving the impression cylinder and initially pull the sheet into contact with the net covered cylindrical support surface of the transfer or delivery cylinder, this bending of the sheet causing the sheet to initially rub against the net until the net has attached to the sheet rearwardly of the sheet leading or gripper end.
In an effort to solve the problem of marking on the gripper end portion, attempts have been made to reduce sheet bend by making the overall diameter of the cylinder smaller so as to increase the clearance between the nip of the impression cylinder and that of the transfer cylinder. While this solution helps reduce marking on the gripper end portion, the increased clearance has been found to induce sheet "flutter" and "slap" at the tail end portion of the sheet with the result that marking or marring of the sheet occurs in the area of the sheet tail end.
"Flutter", which is a vibrational fluctuation of the sheet, typically has been found to occur when printing sheets having thickness in the approximate range of 0.010 inches (0.25mm) down to 0.002 inches (0.05mm), and is believed to be caused by the release of sheet tension as the tail end of a sheet leaves the nip of the impression cylinder and transfer or delivery cylinder. This release of tension is believed to induce a vibrational fluctuation or flutter of the sheet tail end portion causing marking or marring as the sheet flaps against the surface of the transfer or delivery cylinder until the flutter is damped and the sheet attaches to the net.
"Slap", which is a spring back effect caused by release of the tail end of the sheet bent around the impression cylinder as it leaves the nip of the impression cylinder and .the transfer or delivery cylinder, has typically been found to occur when printing sheets having thicknesses in the approximate range of 0.010 inches (0.25mm) up to 0.034 inches (0.86mm). This spring back of the sheet to its original unbent condition upon leaving the nip causes the tail end portion to slap against the surface of the transfer or delivery cylinder causing marking and marring.
Another suggestion that has been made for solving the problem of marking in the gripper end portion of the sheet is that set forth in U.S. Pat. No. 4,690,054 issued Sept. 1, 1987 and assigned to M.A.N. Roland Druckmaschinen Aktiengesellschaft of the Federal Republic of Germany. That prior art patent describes a transfer or delivery cylinder (referred to in that patent as a "drum") wherein the fabric covered drum outer contour is constructed in such a manner that the front or leading edge portion is in the form of a smooth and continuously increasing spiral which merges into a fixed drum radius so that the fabric covered drum surface is contacted by the freshly printed surface of the sheet on the spiral surface first substantially flatly behind the front edge of the drum and then in any subsequent angle of rotation of the drum without the sheet tangent ever being bent or creased.
While the solution suggested in the aforementioned M.A.N. Roland patent may help solve the problem of sheet marking or marring in the area of the sheet leading or gripper end, it does not address the problem of sheet flutter or slap causing marking in the area of the sheet center or trailing edge. Moreover, the construction of a cylinder having the outer contour suggested by this patent is quite difficult and expensive to manufacture. It is also believed that due to the inherent nature of the increasing radius of the spiral portion of the cylinder, the sheet may actually be bent more on initial contact than would be the case with a right circular cylindrical surface, thereby resulting in an increased likelihood of sheet marking or marring rearwardly of the leading or gripper end. That is, the increasing radius spiral just moves the point of sheet marking rearwardly toward the trailing end.
Thus, there exists a need for a transfer or delivery cylinder system which will insure that the freshly printed sheet will be transferred from the impression cylinder without excessive bending of the sheet on initial contact with the support surface of the cylinder, yet will not cause the center or trailing end of the sheet to flutter or slap against the cylinder, thereby to prevent marking on either the leading, center or trailing edges. Further, there exists a need for such a transfer or delivery cylinder which is relatively simple in design, easily made, and economical to manufacture. As will become apparent hereinafter, the present invention solves this need in a novel and unobvious manner.